The present invention relates to papermaking and/or the use of recycled paper/paperboard, and also relates to preserving the starch content of pulp during paper and packaging manufacturing, and further relates to controlling calcium precipitation and/or scaling in the treatment of waste water effluents.
Recycling is a major factor in the modern green economy and is particularly significant in the paper industry's goal to become more efficient and environmentally sustainable. Use of recycled pulp in paper and packaging manufacturing, however, presents several obstacles to achieving high-quality product. Recycled pulp, for example, derived from old corrugated cardboard, writing/printing grades sized or coated with starch, contain starch, usually at high levels, and are beneficial to making recycled paper/paperboard. The grades of paper/cardboard to be recycled also have beneficial levels of CaCO3. Unfortunately, this starch content can become substantially degraded during manufacturing as can supplemental starch added during the manufacturing. Less starch in the resulting product means a loss of or lower mechanical properties in the paper/paperboard product made from the pulp. Further, the calcium found in pulp, such as pulp from recycled sources, can cause calcium precipitation, scaling or fouling in the post treatment of process water that occurs after pulp removal.
More specifically, in the past, numerous papermaking plants (especially those using recycled paper) have experienced numerous problems that may have been related to bacterial problems. However, when the standard approach to combating bacteria was attempted, no success was achieved. Biocides typically used in the papermaking industry and/or typical biocide treatments did not solve the following problems that were being seen in many machines making packaging papers. Those in the industry could not understand what was the exact problem and could not determine a solution to the problem. The present inventor however, determined that a particular microbial activity initiated the following sequence:                Microorganisms release amylases into the papermaking system.        These extracellular enzymes degrade starch into glucose oligomers (e.g., maltose) and glucose (the starch for instance, comes from waste paper, broke and/or wet end additive starch).        The oligomers and monomers are taken up by bacteria and are fermented producing volatile fatty acid (VFAs).        VFAs decrease the process pH (from 7 or higher to 6.5-6 or lower).        The fermentation process is accompanied by an increase in conductivity and decrease in redox potential.        The pH in localized areas around fermenting bacteria can be as low as 1 to 4.        Low pH dissolves calcium carbonate filler (for instance as present in waste paper) into soluble calcium Ca2+ (and CO2).At this point, the problems only get worse: (1) increased growth of fermenting bacteria results in increased production of extracellular amylases; (2) any starch added for strength in the wet end (between mixing chest and head box) is degraded; (3) the glucose oligomers encourage even more growth of microorganisms and so increase slime and/or other bacteria problems; (4) VFAs are a cause of serious odor problems in the paper as well as in the production environment and/or in the surroundings, potentially including habitation areas; (5) when calcium carbonate is dissolved and stabilized by VFAs, filler (e.g., from waste paper) is lost, essentially a loss of raw material; and/or (6) dissolved calcium can cause deposit or scaling problems.        
In mills utilizing anaerobic digesters for waste water treatment, another related problem can develop, as follows:                VFAs react to stabilize dissolved calcium (as VFA-Ca salts) and carry that calcium into the waste water treatment system.        With conversion of VFAs into CH4 and CO2 and a pH increase in the anaerobic digester, calcium scale forms; if excessive scale is formed this can shut down the waste treatment system, which in turn could cause the shutdown of the mill for cleaning of the anaerobic digester.        
In mills utilizing aerobic digesters or ponds for waste water treatment, alone or in combination with anaerobic digesters, problems with CaCO3 fall-out can develop, as follows:                VFAs react to stabilize dissolved calcium (as VFA-Ca salts) and carry that calcium into the waste water treatment system.        With (further) degradation of VFAs in the effluent and with further pH increase during the effluent treatment, calcium carbonate can precipitate resulting in scaling (aerobic reactors) or excessive sludge formation (in aerobic ponds). This can lead to increased downtime for maintenance and cleaning and can cause significant cost for the disposal of CaCO3-rich sludge as chemical waste.        
The present inventor was the first to understand the root of the problem and how to prevent and/or control this problem. The approach used by the present inventor, as described herein, is to reduce or prevent the breakdown of starch by microbiological activity in the paper mill. The root causes of the problem are in summary:                The bacteria and the amylase enzymes produced by the bacteria in the paper machines, because amylase enzymes are very efficient in breaking down starch, for instance, to maltose and glucose.        The stimulation of fermentation metabolism of facultative anaerobic bacteria by high glucose and sugar content in the process water resulting in high levels of VFAs being produced which are at the origin of the solubilisation of calcium filler in the process.        
Thus, the present inventor determined that the best way to solve this problem was to get ahead of the problem and stop the terrible chain of events that are detailed above.